Under the vast undulating surface of the ocean, "constellations" of towering subsea mountains dot the Earth – and they are teeming with sharks.

Beneath the waves, ocean currents roar like the wind over the summits of long-extinct volcanoes. These seamounts rise steeply from the seafloor, soaring to heights of at least 1,000m (3,300ft). Some are pitted with craters, or lined with ridges. Others are topped with large, flat plateaus. Sometimes the peaks can even peep above the ocean's surface, forming islands.

Seamounts are borderlands. Here, on the edge of the deep, reef-dwellers mingle with open-water species at every level of the food chain, from planktivorous fish to top predators.

Each seamount is unique and teeming with life. They bustle with corals, crustaceans, sponges, sea stars, fish, octopus, turtles, whales, dolphins, sharks and more. Compared to the flat seafloor, seamounts host a higher number and diversity of living creatures, many found nowhere else in the world. And there are likely many more yet to be discovered.

There are thought to be more than 100,000 undersea mountains spread across the globe – from the frozen waters of the North Atlantic to the abyssal depths of the tropical Pacific – but fewer than 0.1% have been explored.

"An unprecedentedly large number of volcanic seamounts have been found on the seafloor over the past couple of decades," says Ali Mashayek, associate professor of climate dynamics at the University of Cambridge in the UK.

And today, as ocean exploration accelerates, more and more seamounts are being discovered. During one recent expedition to explore seamounts of the Atlantic Ocean, scientists discovered an unusual number of the ocean's top predators: sharks.

"It was like living in a small village in the middle of nowhere," says Sam Weber, who worked on the island of Ascension for seven years as its principal conservation scientist. "You know everyone. Not huge amounts to do, but good beaches, good diving, good walking."

Ascension Island is "a tiny dot of green" halfway between Africa and Brazil, a lone speck of land in the tropical mid-Atlantic. But look beneath the waves of the seemingly endless ocean that surrounds the isle, and a different story emerges.

Ascension is the tip of an undersea volcano that has "breached the surface and become an island", says Weber. It is just one in a chain of seamounts that stretches for hundreds of miles, "through St Helena and all the way to Africa – right across the Atlantic".

Weber, now a lecturer in marine vertebrate ecology and conservation at the University of Exeter in the UK, explored three seamounts roughly 300km (186 miles) off the coast of Ascension Island. The summit of the Harris Stewart seamount is deep, only reaching as high as the twilight zone, the global layer of ocean which lies between the "sunlight" and "midnight" zones. At about 200 to 1,000m (650 to 3,300ft) depth, the twilight zone is bathed in perpetual dusky light.

The sister "Southern Seamounts" of Grattan and Young, on the other hand, are relatively shallow. Their peaks rise to around 100m (330ft) shy of the surface, and lie just 80km (49.7 miles) apart. These were "a very different place", says Weber.

Alongside the British Antarctic Survey's James Clarke Ross research vessel, Weber spent most of his 16 days at sea aboard a small longline fishing boat, catching and tagging sharks. "We were working with a crew of commercial fishermen – who were brilliant. They handled the catching very effectively, and then we did the tagging work. We all worked together, cooked together, ate together, slept together."

It was on the Southern Seamounts that the Weber and his colleagues found a whopping 41 times more sharks, in terms of biomass, than out in the open ocean.

Compared to surrounding oceanic waters, Weber and his team found the Southern Seamounts supported five times higher diversity, and 30 times higher biomass, of sharks and large predatory fishes combined. There was also a significantly higher number of several threatened and near-threatened species – including silky sharks – and commercially exploited species such as yellowfin and bigeye tuna.

"Sharks, including deep-sea and migratory sharks, love to be around seamounts – across all the global oceans," comments Lydia Koehler, an expert in ocean governance and associate lecturer at Plymouth University School of Biological and Marine Sciences in the UK.

Why seamounts attract so many top marine predators, though, remains somewhat of a mystery.

It is thought animals may visit seamounts for shelter, cleaning or food. Or they could use them as landmarks for navigation.

A seamount forms when volcanic rock, containing tiny iron atoms, cools and solidifies. The iron atoms – which become aligned with the local magnetic field of the Earth – are frozen in time and the seamount's

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